Just being Devils Advocate here , but each arrest takes at least two officers away . So if say 9 arrests were made , I would imagine that would probably wipe out most of the police in the ground.
As fans have said , there was a nasty atmosphere in the ground and the potential was there for it to have escalated after the final whistle. It’s possible that the police officer in charge felt that he needed the cops available to prevent any further disorder outside.
I still can’t understand why those in the North Stand tunnel were not sent up to try and prevent the missile throwing like.